1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus, and an image processing method. In particular, the present invention relates to an image processing for determining an amount of color material for printing in generation of print data of a monochrome image.
2. Description of the Related Art
An ink jet printer image using a plurality of colors of inks as color materials has been widely known as an image format ion apparatus for outputting an image. An electro-photographic type printer has been also known that uses toner as color material. These image formation apparatuses use three colors of cyan (C), magenta (M), and yellow (Y) or four colors of cyan (C), magenta (M), yellow (Y), and black (K) to represent various colors for printing by the subtractive color mixing.
The image formation as described above, however, often causes a situation where, when the printing is performed based on the respective signal values specifying amount of color materials such as C, M, and Y for example, colors intended by these signal values cannot be reproduced faithfully. For example, when sizes of dots formed by the respective color materials are slightly different from one another on a printing medium such as a paper, colors in a printed image composed of the collection of these dots may be observed as the ones slightly dislocated from intended one. This is caused, for example, when amounts (volumes) of ink droplets ejected from printing heads are slightly different depending on individual differences of heads or when the sizes of dots of a latent image formed on a photosensitive material are slightly different from one another in an electro-photographic type image formation apparatus. The slight difference in dot size is also caused by a relation between the type of a printing medium and the characteristic of color material (e.g., ink, toner). Furthermore, the dot size also changes due to the change of these image formation apparatuses with age.
As described above, a phenomena in which an actual color of a printed image is represented to have colors in a color space that are dislocated from colors (position coordinates) intended by a color material signal may be caused in many image formation apparatuses. Herein, such a phenomenon will be referred to as “color deviation”.
Conventionally, so-called calibration has been known as a method to cope with this color deviation. For example, patches are printed by a target printer. Then a color conversion table, a gamma correction table or the like is changed or generated based on the colorimetry result of the patches to adjust amount of color materials to suppress the color deviation. The adjustment of color material is also performed by measuring volume of ink droplets ejected from individual printing heads to change an image processing in a similar manner for example.
However, when color deviation is caused in an image represented by black or gray as achromatic colors, such as monochrome image, it is relatively difficult to adjust the color deviation. Conventionally, gray of particularly low density has been frequently represented by superposing basic three colors of C, M, and Y at substantially the same amount (e.g., see Japanese Patent Laid Open No. 2000-198227). In this case, even a small change in amounts of color materials of the respective colors distorts the balance among the three colors, causing a relatively large shift of a hue. This makes it difficult to adjust the amount of the color materials itself. Furthermore, even a small change of the size of formed dots causes a significant change of the colors due to the same reason. This color deviation in gray means that a color of a chromatic color is slightly visible in the achromatic color, and thus the color deviation is noticeably observed.
FIG. 1 shows the contents of look-up table (LUT) for a color conversion described in Japanese Patent Laid Open No. 2000-198227 for a case where a gray image is printed. The horizontal axis represents 0 to 255 density levels (density values) represented by the respective 8 bit input data of R, G, and B for example for colors on a gray axis in a color space while the vertical axis represents output signal values (0 to 255) of the respective color inks, that is, ink amount (color material amount)) in order to express the respective density values. As shown in FIG. 1, gray is expressed by the three color inks of C, N, and Y in a range from a low density region to an intermediate density region. Specifically, output values of the respective three color inks shown in FIG. 1 are determined as gray having no color deviation in a predetermined color space. When the input density level exceeds about 176, the use of black ink (K) is started and the output signal value at the highest density level is about 128.
FIG. 2 shows another example of a conventional color conversion LUT. FIG. 2 shows, as in FIG. 1, ink amounts of the respective colors to express the colors of the gray axis in a color space. The example shown in FIG. 2 shows the color conversion LUT for a case where, in addition to cyan (C) ink and magenta (M) ink, light cyan (lc) and light magenta (lm) ink for which colorant such as dye has a lower density are used.
Recently, a high quality image comparable to an image by the silver halide photography has been required in the ink jet printer field. One of the major problems in this case is granular quality provided by a printed image to an observer. This granular quality is so-called visual roughness given to an observer when dots formed on a printing medium are conspicuous at a level that can be visually recognized. In order to reduce such granular quality, a plurality of types of inks for similar colors are used that have colorants having different densities as described above.
As shown in FIG. 2, in a low density region, inks of three colors of lc, lm, and Y are used to express gray. A process where the density is gradually increased from a low density causes discrete formation of dots. Thus, the inks having a lower density are used to reduce the granular quality. Output values of the respective three inks in this example are also determined as gray having no color deviation in a predetermined color space. In a region of an intermediate density, output values of lm and lc are close to the maximum value and thus the combination of these inks cannot express a further higher density. This density region also causes a great number of dots to be closely arranged on a printing medium, thus suppressing the granular quality due to individual dots from being conspicuous. Thus, C and M, and further K can be gradually added when this density region is approached to increase the density while suppressing the granular quality from appearing. At the same time, the output values of lc, lm and Y are gradually decreased. Finally, the output value of K has a value higher than those of the other inks, thus providing an expression of gray or black having a superior tone.
However, the methods for determining the ink amount described with reference to FIG. 1 and FIG. 2 as described above have the difficulty in adjustment of the color deviation of a monochrome image of gray or black as described above and also cause conspicuous color deviation.
In addition to these methods, Japanese Patent Laid Open No. 2000-198227 describes a mode for printing black characters or the like in which, in order to express a color of the gray axis, black (K) ink is used in whole regions from a low density region to a high density region. When the granular quality is not required to be particularly considered, gray or black can be expressed by using K ink in whole density regions. This can suppress the color deviation due to unbalance in a case where gray is expressed by the three colors of C, M, and Y for example.
However, the above case may cause the color deviation where an unintended color appears in a monochrome image due to a characteristic owned by K ink itself or a relation between the K ink and a printing medium.
Furthermore, Japanese Patent Laid Open No. 2000-198227 does not suggest the use of K ink for whole density regions for colors other than those of the gray axis. Specifically, such a monochrome image also may be required that has some color tone, such as cold tone, warm tone, other than a pure black tone having a perfectly neutral color tone. This allows various monochrome images according to user's preference to be printed. In this case, it is desirable that a color adjustment is executed for the monochrome images so that monochrome images free from the color deviation and having adjusted color tone can be printed. However, it is clear that this request cannot be satisfied by only the technique as described in Japanese Patent Laid Open No. 2000-198227 for using K ink in whole density regions for colors of the gray axis.